Marcin Kasprzak tasered Michelina Lewandowska before tying her up, gagging her and burying her in woodland near their home.
She used her engagement ring to dig herself free, before eventually flagging down a passing motorist.
She is believed to have spent about an hour underground after Kasprzak had placed a 40kg tree branch on top of the cardboard coffin.
The sentencing judge, Recorder Peter Collier QC told Kasprzak: “The death you intended would have been long and slow.
“It is mind-numbingly awful to imagine the sort of death you intended her to die.”
Kasprzak was found earlier found guilty of attempted murder.
Leeds Crown Court had been told that he was bored with their relationship and had wanted to ensure he gained custody of their three-year-old son.
His friend Patryk Borys, 18, was sentenced to four-and-a-half years in a young offenders institute for helping to kidnap Lewandowska.